The petroleum industry is increasingly turning to heavy crudes, resids, coals and tar sands as sources for feedstocks. Feedstocks derived from these heavy materials contain more sulfur and nitrogen than feedstocks derived from more conventional crude oils, requiring a considerable amount of upgrading in order to obtain usable products therefrom. The upgrading or refining generally being accomplished by hydrotreating processes, i.e., treating with hydrogen of various hydrocarbon fractions, or whole heavy feeds, or feedstocks, in the presence of hydrotreating catalysts to effect conversion of at least a portion of the feeds to lower molecular weight hydrocarbons, or to effect the removal of unwanted components, or compounds, or their conversion to innocuous or less undesirable compounds.
Hydrotreating is well known in the art and typically requires treating the petroleum streams with hydrogen in the presence of a supported or unsupported catalyst at hydrotreating conditions. Supported catalysts are usually comprised of at least one Group VIB metal with one or more Group VIII metals as promoters on a refractory support, such as alumina. Hydrotreating catalysts that are particularly suitable for hydrodesulphurization, hydrodearomatization, as well as hydrodenitrogenation, generally contain molybdenum and/or tungsten promoted with a metal such as cobalt, nickel, iron, or a combination thereof. Cobalt promoted molybdenum on alumina catalysts are most widely used when the limiting specifications are hydrodesulphurization. Nickel promoted molybdenum on alumina catalysts are the most widely used for hydrodenitrogenation, partial aromatic saturation, as well as hydrodesulphurization.
Unsupported mixed Group VIII and Group VIB metal catalysts and catalyst precursors used for hydroconversion processes are known in the art as disclosed in U.S. Pat. Nos. 2,238,851; 5,841,013; 6,156,695; 6,566,296 and 6,860,987, amongst others.
Hydrotreating catalysts based on group IIB metals such as zinc were one of the first base metal hydrotreating catalysts invented, and were described in U.S. Pat. Nos. 1,922,499; 1,932,673; and 1,955,829. However, U.S. Pat. No. 4,698,145 teaches that group VIB metals based catalyst exhibit performance superior to group IIB metals based catalysts. Hydrotreating catalysts based on group IVA metals such as tin or lead were described U.S. Pat. Nos. 4,560,470 and 5,872,073.
Unsupported mixed Group IIB and Group VIB metal catalysts and catalyst precursors are known in the art. Methods for making catalyst precursors and catalyst precursor compositions in the form of oxides of a Group IIB metal and molybdenum and tungsten are taught in, for example, U.S. Pat. Nos. 1,932,673 and 1,955,829. Sulfided hydrogenation catalysts of molybdenum and tungsten are also known. U.S. Pat. No. 4,698,145 teaches the process of making a sulfided catalyst with ammonium thio salts of Group VIB metals such as molybdenum or tungsten and salts of zinc in the presence of a nitrogen containing additive. Unsupported mixed group IVA and group VIB metal catalysts and catalyst precursors are also known in the art. These are made from the chlorides and sulfides in a multistep synthesis as described in, for example, U.S. Pat. Nos. 4,560,470 and 5,872,073.
As the environmental impact of effluents or water disposal from industries has become increasingly scrutinized, there is a need to limit the use of toxic materials to the greatest extent possible. In the process of making catalyst precursors in the prior art, chelating agents such as ethylene diamine(tetra)acetic acid (EDTA), hydroxyethylene diamine triacetic acid, and diethylene triamine pentaacetic acid, etc. are employed. These materials are far from environmentally benign.
Under the reaction conditions employed in hydrotreating processes, catalyst performance, over time on stream, tends to become fouled with carbon deposits, especially when the feedstock includes the heavier, more refractory fractions of hydrocarbon, S and N species in the heavier crude oil. The accumulation of such deposits tends to reduce the catalyst activity. Thus, catalyst average temperature (or C.A.T.) needs to be raised gradually in order to maintain product quality, such as the N concentration in the upgraded product. The rate of C.A.T. being raised per unit time is defined as the fouling rate of catalyst.
Catalyst performance depends on a number of factors. For some catalysts, an important factor is the partial pressure of hydrogen employed in the process. A low pressure process can be generally described as having a pressure of less than 600 psig, and in one embodiment, between 400 to 600 psig. In a very low to low pressure hydroconversion process, some unsupported multi-metallic catalysts in the prior art have relative activity that is about ˜⅓ of the activity at moderate to high pressure process (2000 to 3000 psig and elevated temperatures generally ranging upward from 650° F.). Multi-metallic catalysts in the prior art are not suitable for use in low pressure reactors of 300-400 psi due to their low activity.
There is a need for improved hydrodesulfurization (HDS), hydrodearomatization (HDA) and hydrodenitrogenation (HDN) catalysts having the appropriate morphology, structure, and optimum catalytic activity for high yield conversions of lower grade hydrocarbon feedstocks to higher value products. There is a need for a process for making such improved catalysts. There is still a need for chelating agents in the manufacture of catalyst precursors that are less toxic or more environmentally friendly or biodegradable without impairing performance in hydroprocessing catalysis. There is a need for catalysts with improved fouling resistance characteristics. There is also a need for catalysts that perform satisfactorily even in low pressure hydroconversion processes.